Protection of metals from oxidation (rusting) and subsequent corrosion is often vitally important, such as, for example, when such metals are used to construct components incorporated into automotive, aerospace, architectural, and other industrial structures and parts. Various methods have been employed to achieve varying levels of corrosion protection.
In some cases, a galvanization process is used to impart corrosion protection to metallic surfaces. This process involves the hot-dip or electroplating application onto a metal substrate of a metal film deposited from a metal ingot. The metal of the metal film often has a greater ionization tendency than the metal of the metal substrate. As a result, as long as physical contact is maintained between the metal film and the substrate, the film is theoretically preferentially oxidized while the underlying substrate, which acts as an electrical conductor to transfer electrons from the metal film to oxygen, is protected.
Galvanization, however, is not ideal in all situations. For example, when utilizing hot dip galvanizing, it is difficult, if not impossible, to control the thickness of the metal film. As a result, hot dip galvanizing is not usually suitable in cases where corrosion protection is required for relatively small metal articles with complex shapes, such as fasteners, for example, nuts, bolts, and the like. Electroplating galvanization, on the other hand, while often enabling improved film thickness control over hot dip galvanizing, can be an expensive process due, for example, for the need to prevent “hydrogen embrittlement.” This phenomena is known to occur during the plating process, wherein hydrogen is absorbed into the coated metal article and entrapped. Subsequently, the hydrogen can cause failure. As a result, additional, costly process steps are often employed to minimize or prevent hydrogen embrittlement.
In some cases, metal substrates are protected by use of corrosion-resisting primer coatings that incorporate metal particles, often zinc, as a metallic pigment. These coating compositions produce a coating that utilizes the same mechanism for corrosion protection as the metal films resulting from galvanizing. Often referred to as “zinc-rich primers”, such coating compositions often outperform galvanization and are commonly applied to a metal substrate by a dip spin procedure. These compositions often incorporate zinc particles, often zinc flake, as the metallic pigment in combination with an organic binder, such as an epoxy resin and/or an inorganic binder, such as a silicate.
While “zinc-rich primers” developed heretofore are suitable in many applications, they do have certain drawbacks that can render them deficient in some cases. For example, to be effective, it has been believed that these compositions should deposit a continuous layer of metallic pigment, such as zinc, onto the metal substrate. When a powder, which is relatively inexpensive, is used, it is often important to apply the composition at relatively large film thickness, usually greater than 3 mils (76.2 microns), to ensure that a continuous layer of metallic pigment is deposited. The use of such thick films is, of course, undesirable from a cost standpoint. It can also render the use of such compositions impractical when corrosion protection is required for relatively small metal articles with complex shapes, such as fasteners, for example, nuts, bolts, and the like.
As a result of this perceived deficiency, metal flakes, such as zinc flakes, are often used as the metallic pigment in zinc-rich primer compositions. The use of these thin, plate-like structures, can result in the deposition of a continuous film of metallic pigment, even when the composition is deposited at a relatively low film thickness, even below 1 mil (25.4 microns). The nature of these materials, however, often causes the resultant coating to exhibit poor adhesion to a metal substrate as well as subsequently applied coatings. Thus, up to four dip applications of a solvent based colored coating composition is often applied over the primer (black is often a desired color). Moreover, aqueous based, electrodepositable coating compositions, which are often desirable for use as corrosion inhibiting coating compositions, often do not adhere to zinc-rich primers that rely on the use of commercial zinc flakes.
A disadvantage that has been observed in the use of inorganic binders in zinc-rich primer compositions is that they tend to be brittle and, therefore, the resulting zinc-rich primer composition can be powdery and exhibit poor adhesion to the metal substrate. This deficiency is particularly problematic when attempting to coat small parts, such as fasteners, which are handled in bulk. In this process, the parts often contact one another. As a result, when a brittle, poorly adhered film is applied to the parts, the film is easily damaged when the parts contact one another during the coating process. This damage leads to poor corrosion resistance performance.
As a result, it would be desirable to provide coating compositions that can impart desirable levels of corrosion protection to metal substrates even when applied at relatively low film thickness. Moreover, it would be desirable to provide such coating compositions that are flexible and adhere well to metal substrates as well as a subsequently applied aqueous electrodepositable coating compositions, to provide a desired color and a desirable level of corrosion protection to a metal article, such as small metal parts with complex shapes, such as fasteners, for example, nuts, bolts, and the like.
As previously mentioned, small metal parts, such as those mentioned above, are often handled in bulk during the coating process. In other words, many parts are coated simultaneously in a coating apparatus. In many instances, as indicated earlier, the individual parts come on close proximity to or contact each other while passing through the coating and drying systems such that when the coating is applied over the parts and dried or cured, two or more parts may adhere together at the point of engagement (often referred to as a “touch point”). These coated parts must then be separated from each other with some degree of force that, typically, results in the removal of at least some of the coating from each of the parts at or around the touch point. If a light, silver-colored (as is common) zinc-rich primer has been deposited under a dark, often black-colored (as is common) electrodeposited coating, removal of the dark electrodeposited coating will reveal the presence of the light undercoating, which is often unsightly and undesirable. As a result, it would be desirable to provide zinc rich coatings that have the flexibility and adhesion characteristics described above and which are dark in color, such as black, so that they exhibit a color similar to that of a subsequently applied electrodeposited coating.